AQUATIC PLANTS – back to the water

The many thousands of plant species which today cover the land surface of the earth have all evolved from ancestral forms which were aquatic. Indeed, life itself had an aquatic origin. Land plants have thus adapted to life out of water and this is reflected in their structure and, especially, their water relations and their methods of reproduction. But if we look closely at the aquatic plants of today they show traces of features such as cuticles and stomata which are characteristic of land plants — why? It is now known that these are plants which are secondarily aquatic, that is to say they are descended from land plants and have subsequently invaded the water.

Aquatic plants are found in most of the major taxonomic groups, ranging from minute planktonic and surface-dwelling algae, through a relatively small number of species of mosses and ferns to the many species of flowering plants (angiosperms) which comprise most of the aquatic plants. There are no truly aquatic gymnosperms.

Apart from the algae, aquatics range in size from free-floating forms less than imm (0.04m) across to full trees, although most are her- baceous. The re-invasion of water has in many cases resulted in marked alteration and reduction in the morphology of the plants, so that it is sometimes hard to determine the taxonomic relationships of different species.

To sink or swim—types of water plants

If the main problems usually faced by land plants include those of support and lack of water, the environmental stresses on aquatic plants are nearly all due to ‘too much’ water. The depth of the water and the strength of the current will profoundly influence the type of plant that can grow at a given site. The physical density of plant tissue is quite close to that of water, so that plants that are fully immersed have few problems of support, unlike land plants living with their foliage in the air. The water plant must, however, be able to withstand the stresses imposed by the flow of water. In most cases this is achieved by having highly flexible stems and leaves which will readily bend into a shape that offers least resistance to the water flowing by, but which are strong enough to resist the pull of the water.

The degree to which the plant is immersed in the water will obviously influence the effects of water flow, and water plants can be divided into three groups: submerged, floating and emergent, depending on the position of their leaves relative to the water surface. Submerged water plants Submerged water plants are those whose vegetative parts are normally beneath the water at all times (although most still produce aerial flowers). Virtually all of these plants are rooted in the substrate, although a few, such as the horn-wort, Ceratophyllum, have no roots and float freely below the surface. Submerged plants usually have one of two growth forms. Firstly, as a rosette with the leaves all arising from a short rhizome or root stock, as in the shore-weed, Littorella, the cape pondweed, Aponoge-ton, or in the quill-wort, Isoetes, a distant relative of the ferns. In the second type the leaves are borne along long flexible stems. Some of these may root in several places. Typical of this group are the pondweed, Potamogeton, and the Canadian pondweed, Elodea canadensis. A similar growth form is found in submerged aquatic mosses such as Fontinalis.

The leaves of submerged plants are frequently elongated or linear in shape. Potamogeton com-pressus has leaves io-20cm (4—8in) long by only 2-4mm (0.08-0.16in) wide, whilst in Elodea the leaves may measure 10mm by 2-]mm (0.4 by 0.08-0.12in). Less elongated submerged leaves are often oval or lance shaped, as in P. lucens. Dissection of submerged leaves occurs in many species, such as the water milfoil, Myriophyllum verticillatum, and the water crowfoots, species of Ranunculus. Here, the leaves, which may be opposite ,or whorled in their arrangement on the stem, are divided into numerous thin strands, sometimes as many as 40 or 50. The advantage to the plant of linear or dissected leaves probably lies in the fact that they will offer relatively little resistance to flowing water and are less likely to tear, compared with broad leaves.

The internal structure of submerged leaves and stems reflects the nature of the aquatic environment. With little need for support there are few fibres or strengthening tissues within the stems and few woody, or lignified, vessels in the xylem. In many submerged plants the whole vascular system, especially the xylem, is markedly reduced compared with that in land plants, sometimes with the complete loss of xylem vessels and their replacement, as in Potamogeton, by a cavity.

Another striking feature is the presence of many cavities, or lacunae, within the tissues. These elongated cavities are found in roots, stems and leaves and in some species may occupy the bulk of the tissue volume. Their number and arrangement are often characteristic of a particular genus or species. For example, Isoetes has just four large lacunae running along each leaf, whereas the stem of Myriophyllum has a ring of lacunae around the central vascular strand and the leaves of Potamogeton appear to consist of a network of lacunae when viewed in cross-section.

Perhaps the most extraordinary group of submerged plants are those which belong to the family Podostemaceae. They are all tropical or subtropical and their structure is so reduced as to form a thallus with no true stem or roots. The thallus, which is sometimes branched, may be floating or attached to rock surfaces and in some species bears a variety of lobed or dissected leaflike structures. Although clearly flowering plants the evolutionary relationships of this family are most obscure. Floating water plants

Whilst most submerged plants may remain more or less hidden beneath the water those water plants whose leaves float on the surface are more often noticeable due either to the beauty of their flowers, as with the water lilies, or to their presence as disruptive weeds blocking waterways and lakes, as with Salvinia and Pistia.

The environment of a floating-leaved plant is in some ways even more specialized than that of the submerged plant. While the lower surface of the leaf is immersed in water, the upper surface is exposed to the rigours of the aerial environment. The effects of wind and waves will be particularly pronounced, wind tending to lift and tear the leaves while waves may tear or swamp them. It is thus not surprising that floating vegetation tends to be found on relatively sheltered waters. Floating leaves are often oval, as in Potamogeton natans, or near- circular in outline, as in the water lilies, Nuphar and Nymphaea. A circular leaf form may be less likely to be swamped by waves and less likely to tear. The extreme example of this form is shown by the giant Amazonian water lily, Victoria amazonica, whose leaves may reach 1.5m (5ft) in diameter and possess an upturned rim more than 15cm (6in) high. The huge leaves are reinforced underneath with a system of stiff ridges and are so buoyant that in the mid-nineteenth century Joseph Paxton was able to float his young daughter on such a leaf! When water does fall on the leaf surface, from waves or rain, its displacement is often aided by the presence of a waxy water-repellent cuticle on the upper leaf surface. This cuticle will also be important in reducing the evaporation of water from the leaf tissues, and in this respect the upper surface of many floating leaves is similar to that of land plants, with functioning * stomata. Unlike land plants, however, chloro-plasts are often found in the epidermal cells of the upper surface. The lower leaf surface is usually without stomata, although relict nonfunctional stomata may be present in some plants, such as Potamogeton. The cuticle on the lower surface is often absent or relatively thin. Lacunae, like those of submerged plants, are found in many floating leaves, and being gas-filled are important in maintaining buoyancy.

Where floating-leaved plants are rooted on the bottom, the leaf stalks (petioles) usually have a structure similar to that of submerged plants. Rooted plants such as these are rarely found in deep water, although the petioles may be long and trailing. This is especially important if the plant is growing where there may be changes in the depth of water. If the petioles were too short and the water level rose the plant could ‘drown’.

Plants which are free-floating are again usually found on relatively slow-moving and sheltered waters. In such situations they may become extremely abundant, spread rapidly and become weeds of international importance.

Unlike the rooted floating-leaved plants those which are free-floating show a great variety of structure. Some of the simplest are the duckweeds, Letnna. This genus of worldwide distribution has no separation into stem and leaves, but merely consists of one or more platelike thalli usually less than icm (0.4m) across. The thallus, which bears a single root, is kept buoyant by numerous lacunae. Even smaller is another member of the same fimily, Wolffia, the smallest of all flowering plants, a tiny globular thallus with no leaves, shoots or roots, often less than 0.5mm (o.02in) across. Even so, Woljfia, like Lemna, may at times entirely cover the surface of sheltered water bodies.

Many free-floating plants have a rosette type of growth form; the leaves are borne in whorls or spirals on a form of shortened central stem. This is well shown in Pistia stratiotes, an important weed of tropical waterways. The rounded leaves, I-2cm (o.4-o.8in) across, have a deep central ‘keel’ composed of spongy tissue well supplied with lacunae which acts as a float. The upper surfaces of the leaves are covered with dense hairs which are water repellent and prevent the leaf from becoming swamped.

Water-repellent hairs are also found on the upper leaf surfaces of the infamous water-fern Salvinia. This plant bears its leaves, 2-5 cm (o.8-2in) long, in pairs along a short stem. liuoyancy is maintained by air trapped between the hairs on the leaf surface. There are no roots, but much-branched feathery modified leaves hang down in the water below the floating leaves and carry out the same functions as roots. The effectiveness of the flotation mechanism in Salvinia was demonstrated in Central Africa when Lake Kariba was formed soon after i960 by the damming of the River Zambezi. As soon as the lake began to fill extensive colonics of Salvinia appeared on its surface. Growth was extremely rapid, resulting in hundreds of square kilometres of the lake surface being covered by dense mats. The only way Salvinia could have reached Lake Kariba was from swamps upstream on the Zambezi. The Victoria Falls are en route, and Salvinia reaching

Kariba must have survived a drop of over 100m (328ft)!

Originating in Central and South America, Salvinia has since the 1940s become a serious weed, especially in Central Africa and Ceylon. The dense rapidly growing mats of vegetation choke the waterways, making fishing and the passage of small boats virtually impossible. As the old dead leaves decay the oxygen in the water beneath may become depleted, to the detriment of any animal life such as fish originally present. Emergent vegetation

Emergent vegetation, the third type of aquatic plant, may often form extensive stands in relatively shallow water. These are plants which are rooted in the substrate below the water and whose foliage is borne wholly or in part in the air above the water. Structurally this emergent foliage is often little different from that of land plants, with well-developed vascular tissues and with cuticle and stomata on both sides of the leaves. Lacunae may still be present in some species, however, and it should be remembered that the roots, stem bases and young shoots will still have to exist in an aquatic environment.

Emergent plants may be dicotyledons, such as the bog bean, Menyanthes trifoliata, or monocotyledons, such as the common reed, Phragmit.es or the reedmace, Typha. Monocotyledons often form conspicuous dense stands of vegetation at the edges of rivers and lakes, forming a community known as reedswamp. The combined effects of the plant shoots slowing the water currents, causing the deposition of silt, and the accumulation of dead plant remains may lead to the gradual filling of the water body, leading to the formation of drier land bearing different plant communities—a type of succession known as a hydroscrc.

The productivity of many reedswamp communities is high, the stands often having a large total leaf area but allowing an efficient distribution of light down to the lower leaves. In some parts of the world reedswamp plants are of economic importance, as in Roumania, where the vast areas of Phragmitcs in the Danube delta are the basis of a major paper and chemicals industry.

One of the first water plants to be important economically was papyrus, Cyperus papyrus, which was used in Ancient Egypt for building and paper-making, and which is still important locally. Although normally a rooted emergent, papyrus may at times form floating rafts upon which other plants may become established. In Central Africa these rafts are known as Sudd and may form vast areas of virtually impenetrable marshy thickets.

Too much and too little—the aquatic environment

The degree of light penetration down into the water will obviously influence the depth to which submerged plants can grow, the limit usually being at intensities of between I and 3 percent of full sunlight. The depth at which this occurs will depend on the amount of silt and coloured material in the water and on the degree of shading due to aquatic plants and plankton. In some rich or silty lakes the limit to growth may be at a depth of im (3 3 ft) or less, whereas in some clear, unproductive lakes submerged plants may be found down to depths of as much as 10m (33ft).

Oxygen diffuses more than a thousand times more slowly in water than in air, so the supply of oxygen to the submerged parts of water plants may be limited. Indeed, the roots may often be in a virtually oxygenless environment. The presence of lacunae and air spaces in floating and emergent plants will provide a pathway allowing oxygen to diffuse to the submerged parts. It has been shown, for example, that oxygen may diffuse out from the submerged root tips of Menyanthes. Submerged plants have no access to the air, but oxygen deficiencies may be partially relieved by the retention in the lacunae of some of the oxygen released in photosynthesis. This may, in fact, be one of the ways in which lacunae are formed. Nevertheless, many water plants undoubtedly undergo periods when their growth is limited by lack of oxygen.

Mineral ions are taken up by water plants by both the roots and the submerged foliage, and growth may be critically affected by the concentrations of the minerals in the water. In particular most water plants are intolerant of the saline conditions found in salt lakes and estuaries. In such conditions high salt concentrations may disrupt the water balance of the plant or lead to the accumulation within the tissues of harmful amounts of salts, as well as affecting the uptake of necessary minerals. Some plants, such as Pliraqmites communis, can tolerate a wide range of salinities, being found in both freshwater and in estuaries. Others, such as Scirpus maritimus, are nearly always found in brackish or estuarine conditions. Partial immersion in full sea water is tolerated by the trees of the tropical mangrove swamps. The mangrove roots are often in highly oxygen-deficient muds, and have specialized roots, pneumatophores, which project above the mud and water surface enabling air to pass into the root system.

Only a few flowering plants have become adapted to life totally immersed in the sea. These plants, such as Posidonia and Zostera, the eel-grass, flower and spread by seed in the sea—true marine flowers.

Reproduction in the water

In many water plants reproduction is usually vegetative. This is especially true in free-floating plants where their rapid spread could not occur fast enough by means of purely sexual reproduction. Some plants such as Lemna reproduce simply by budding of the thallus, whilst others, such as Pistia, form stolons, spreading horizontal stems on which new plants are formed. Propagation by the simple break-up of the old stems may also occur, as in Salvinia. Vegetative reproduction in rooted plants is often by means of tubers or by underground stems, rhizomes, as in Phragmites and papyrus.

Sexual reproduction in aquatic plants is in most cases similar to that in land plants, especially in floating and emergent plants. Many submerged plants, too, have not entirely lost their links with the air, putting up flower bearing shoots above the water, as in the bladderwort, Utricularia. These aerial flowers are usually either wind- or insect-pollinated. Some water plants may reduce the complexity of the flowers with, for example loss of sepals and petals. Woljfia shows an extreme example of this, the male flower consisting of a single stamen only.

Only a few genera, such as Ceratophyllum and the marine Zostera, have become true water plants in that they flower below the surface. In Ceratophyllum the anthers from the male flower have a small float. When mature they detach and float to the surface where they dehisce, releasing the pollen. The pollen grains sink through the water, and in doing so some of them may come into contact with the elongated stigmas of the female flowers. Thus with water pollination true independence of the air is achieved and the re-invasion of the water is complete.

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